# How will flying through mountains cause altimeter problems?

There is an old warning on mountain flying that wind flowing through a narrow valley will cause Venturi effect, thus causing a drop in static pressure and making our little Cessna fly lower than indicated, possibly hitting some hidden mountain peak. I'm certain I know the answer to this one, but I'd like to hear someone explain it better, so here is my question:

You fly a Cessna 172 level at FL50 (pressure altitude) with a huge 40kts headwind. You need to traverse a narrow valley passing between two huge peaks on either side. Will your airplane fly at a constant true altitude in a straight line, or will the Venturi effect caused by the wind and the valley cause our aircraft to sink while passing through the valley?

.

• I'd be interested to know which valley/mountain system? Mar 17, 2015 at 0:17
• If you try to fly through a mountain you'll have a lot of problems, not just with the altimeter ;) May 24, 2015 at 17:28

You basically answered the question yourself already. Venturi states: the sum of pressures in a system stays constant. The system is the valley and the pressures are the air (static pressure) within the valley and the wind flowing through it (dynamic pressure).

Static Pressure + Dynamic Pressure = Total Pressure If the wind flows faster, we got higher dynamic pressure. Since the sum stays constant the static pressure has to fall as a consequence. Of course, then, all pressure levels shrink under lower static air pressure as they would do under cold temperatures. Flying at a constant pressure level results in following these shrinking levels and eventually end up at a lower true altitude. That's the main reason why your true altitude is lower than your indicated.

If you knew the actual static pressure in the valley, you could adapt your altitude by the rule of thumb: per 1hPa: 30ft

So answering your question: if the venturi effect reduces static air pressure in the valley, the above stated applies.

From my understanding of this, the Venturi effect created in the valley will drop temperatures much like the Venturi effect causes temp drops in your carb. which you know will change pressure (increase) and thus change altimeter reading. Since you are flying at a relatively low altitude you will be affected by the change.

• If the pressure increased, then the altimeter would under read and the aircraft would fly higher than the indicated altitude? Mar 13, 2015 at 20:26
• Primarily, the venturi effect causes a drop in pressure at the peak, proportional to wind speed, causing the altimeter to over-read (show higher than actual) compromising vertical clearance. Mar 24, 2020 at 12:13